This invention relates generally to control systems and more particularly to position detection and collision avoidance systems.
In the aircraft industry, it is necessary to provide access of persons to the outer surfaces of large aircraft to paint and maintain the aircraft. To provide such access to the outer surfaces of large aircraft, a plurality of large gantry cranes, known as stackers or stacker cranes hanging from rails mounted within and near the ceiling of an aircraft hangar, have been employed. Each of these gantry cranes has been operated by an operator riding on a platform on the lower end of a vertically extendible and retractable crane mast.
Employing a plurality of such cranes has presented particular problems with respect to collisions between the cranes themselves, between the cranes and other objects in the hangar and most importantly between the cranes and the aircraft. The problem of collisions between the cranes and the aircraft has been exacerbated by the fact that, in order for the painters/crane operators to effectively prepare and paint the aircraft, the crane platforms need to be positioned within an approximate range of four inches to ten feet from the aircraft, depending on the task.
A single collision between a crane and one of today's modern aircraft can be very costly, not to mention the danger of human injury. Thus, it is extremely important that the gantry cranes not hit the aircraft.
Because of the magnitude of the task of painting a large commercial aircraft, because of the potential for economic loss associated with collisions, and because of the inadequacy of previous crane control systems, the painting state has become a "bottle neck" in aircraft manufacturing.
One reason for the difficulty experienced in developing control systems for avoiding collisions between cranes and aircraft has been due to the fact that each aircraft is not parked in the exact same location and with the exact same positioning and orientation relative to the cranes and the hangar. In other words, it has been difficult to determine the position of the surfaces of the aircraft relative to the hangar and the cranes, and this in turn has made difficult the development of adequate control systems.
Control systems for cranes operable around large aircraft have evolved from the earliest stages where crane operators themselves were alone responsible for manually and visually keeping the crane and the movable platform on the crane away from the aircraft and other objects in the hangar by appropriately operating the crane's controls to avoid collisions. Of course, with such a system, human error and accidental depressions of crane controls have resulted in both minor and major collisions.
A first attempt at solving some of the problems has been termed a collision detection system. With such a system, a number of strategically placed wands, contact tapes and bumpers have been placed around the perimeter of the crane's platform and on the crane structure itself at locations where the platform or the crane could collide with the aircraft or other objects in the hangar. Upon touching or bumping the aircraft or other objects, these systems have, through a system of relays, interrupted control power to the cranes to avoid further and more damaging movement.
Though such collision detection systems often have avoided major collisions, they have had several disadvantages, including the fact that activation of the collision detection wand, contact tape or bumper has cut control power to all stacker crane functions and has required service calls to restart the stackers. This has required as many as 20-30 service calls per week to enable further crane operation. Also, these prior detection systems have damaged wet paint on the aircraft or the aircraft as they have had to touch the aircraft before interrupting power to the crane. Moreover, these prior control and collision detection systems have not been found infallible. There have continued to be both minor and major collisions in the paint hangar.
A second generation of control systems termed anti-collision systems have been developed in an attempt to solve the problems associated with the aforementioned first generation collision detection systems. These anti-collision systems have employed commercially available non-contact sensors, such as ultrasonic and diffused reflective infrared sensors wherein either an ultrasonic or reflective infrared wave has been propagated toward the object or aircraft to be avoided. Upon detection of a reflected ultrasonic or light wave from an object, the non-contact sensors have interrupted motion of the crane. With such systems, the operator has usually been able to manually operate the crane away from the danger, i.e., by pulling the crane upwardly, and if necessary, the operator has been able to manually override the anti-collision system to move the crane away from danger.
While the second generation control system virtually eliminated the need for service calls to repower the cranes after detection of an impending collision, such systems still have had several disadvantages. For example, compound curved surfaces on the aircraft have scattered ultrasonic signals so as not to provide sufficient feedback. Also, differing surface reflectiveness of objects have caused the stacker cranes to stop at varying distances, between 6 and 18 inches from the aircraft, depending on the brightness of the surface. Moreover, certain objects within the hangar have not provided an upper surface effective to stop motion of the crane before a collision has occurred. Still further, in the painting environment, the use of the diffused reflective infrared sensors has become limited, since overspray can cover the light emitting portion of the sensors causing the sensors to become less effective.